1. Field of the Invention
The invention relates to a control apparatus and control method for an automatic transmission. More specifically, the invention relates to a control apparatus and control method for an automatic transmission, which performs neutral control.
2. Description of the Related Art
An automatic transmission mounted in a vehicle includes a shifting mechanism which is connected to an engine via a torque converter and the like, and which includes plural power transmission paths. The automatic transmission is configured to automatically change the power transmission paths based on an accelerator pedal operation amount and a vehicle speed, namely, configured to automatically change speed ratios (shift speeds). Usually, the vehicle including the automatic transmission is provided with a shift lever operated by a driver. A shift position (e.g. a reverse running position, a neutral position, or a forward running position) is selected through an operation of the shift lever, and automatic shift control is performed among plural shift speeds corresponding to the selected shift position (usually, the forward running position).
When the vehicle including such an automatic transmission is at a standstill with the forward running position being selected, driving force from an engine running at idle is transmitted to the transmission via the torque converter and then transmitted to wheels, causing a so-called creep phenomenon. The creep phenomenon is highly useful under a predetermined condition. For example, the vehicle at a standstill on an uphill road can start smoothly due to the creep phenomenon. However, the creep phenomenon is not desired, when the vehicle needs to be kept at a standstill. Accordingly, in such a case, a brake of the vehicle is applied so that creeping force is suppressed. Namely, the creeping force from the engine is suppressed by the brake, causing a problem that fuel efficiency of the engine is reduced.
Therefore, a proposal has been made to improve the fuel efficiency in a state where the vehicle is at a standstill since the brake is applied by depressing a brake pedal and the accelerator pedal is substantially fully released when the forward running position is selected. It is proposed that the transmission be placed in a state close to the neutral state while the forward running position is selected so that fuel efficiency is improved. Namely, it is proposed that generation of the creeping force be prevented by placing the transmission in the state close to the neutral state.
Japanese Patent Application Publication No. JP 05-87236 A discloses a creep control apparatus for a vehicle, which prevents a vehicle from creeping when a predetermined condition is satisfied while the forward running position is selected. This creep control apparatus for a vehicle prevents the vehicle from creeping by achieving the neutral state, if a predetermined condition including a condition that a foot brake is depressed is satisfied, even when a shift range of the automatic transmission is set to a forward running range. The creep control apparatus includes a detection portion which detects an operation amount of the foot brake. A threshold value for the operation amount of the foot brake, which is used when creep prevention control is started, is set to a value larger than a threshold value for the operation amount of the foot brake, which is used when the control returns from the creep prevention control to normal control.
With this creep control apparatus for a vehicle, hysteresis of the operation amount of the brake is set when the creep prevention control is started and when the control returns from the creep prevention control to the normal control. Accordingly, it is possible to prevent the situation where the creep prevention control is started although the driver has no intention to start the creep prevention control. Also, the creep prevention control is started only when the foot brake is depressed by a considerably large amount, making it significantly easy to perform an operation that needs to be performed while the vehicle is running at a speed equal to or lower than a creeping speed. For example, it becomes easy to perform an operation for performing parallel parking, or an operation for placing the vehicle into a garage.
However, in the creep control apparatus for a vehicle disclosed in Japanese Patent Application Publication No. 05-87236 A, a brake hydraulic pressure sensor is used as the detection portion that detects the operation amount of the foot brake so that whether the creep control can be performed is determined. A condition that a hydraulic pressure detected by the brake hydraulic pressure sensor is equal to or higher than a predetermined value is one of the conditions under which the creep prevention control is started. If the brake hydraulic pressure sensor does not operate properly, it is difficult to perform the creep prevention control (neutral control), and cause the control to return from the creep prevention control (neutral control) to the normal control. Particularly, when the driver attempts to start the vehicle by releasing the brake pedal, timing at which the automatic transmission is shifted to the forward running state may be delayed, and the driver may feel a sense of discomfort. If another sensor is provided in addition to the brake hydraulic pressure sensor in order to address such a problem, a production cost increases.